While we have never actually seen the stuff, new experiments may finally be able to detect it.

A number of experiments now underway aim to detect elusive dark matter particles, including those at the Sanford Underground Research Facility in South Dakota and the Gran Sasso underground laboratory in Italy. The idea is that dark matter particles might be observed interacting with ordinary matter, if the ordinary matter is dense enough. So these experiments use vast tanks filled with liquid xenon, which is denser than solid aluminum, as dark matter detectors. The detectors are located deep underground to shield them from sources of radiation that might confound the results.